Steel material having excellent atmospheric corrosion resistance

ABSTRACT

A steel material has excellent atmospheric corrosion resistance and a composition which contains more than 0.06% and less than 0.14% C, 0.05% or more and 2.00% or less Si, 0.20% or more and 2.00% or less Mn, 0.005% or more and 0.030% or less P, 0.0001% or more and 0.0200% or less S, 0.001% or more and 0.100% or less Al, 0.10% or more and 1.00% or less Cu, 0.10% or more and 0.65% or less Ni, 0.0001% or more and 1.000% or less Mo, preferably 0.005% or more and 1.000% or less Mo, 0.005% or more and 0.200% or less Nb, and Fe and unavoidable impurities as a balance.

TECHNICAL FIELD

This disclosure relates to steel structures used mainly outdoors such asbridges and, more particularly, to a steel material preferably used forforming members which are required to have atmospheric corrosionresistance in a high salt environment such as a coastal environment.

BACKGROUND

Conventionally, as a material for forming steel structures used outdoorssuch as bridges, weathering steel is used. Weathering steel is a steelmaterial where, in an atmospheric environment, a surface of the steelmaterial is covered with a highly protective rust layer withconcentrated alloy elements such as Cu, P, Cr and Ni so that thecorrosion rate is remarkably reduced. It has been known that, due to itsexcellent atmospheric corrosion resistance, a bridge built by usingweathering steel can withstand the service for several tens of years ina paintless state in many cases.

However, it has been known that, in an environment where an amount ofair-borne salt is large such as a coastal environment, theabove-mentioned highly protective rust layer is hardly formed so that itis difficult to acquire the practically effective atmospheric corrosionresistance (hereinafter an amount of air-borne salt being a valuemeasured in accordance with JIS Z2382).

According to Japanese Patent No. 3785271, conventional weathering steel(JIS G3114: Hot-rolled atmospheric corrosion resisting steels for weldedstructure) can be used in a paintless state only in regions where anamount of air-borne salt is 0.05 mg·NaCl/dm²/day (hereinafter, unit(mg·NaCl/dm²/day) also being expressed as mdd in some cases) or less.Accordingly, in an environment where an amount of air-borne salt islarge such as a coastal environment, an ordinary steel material (JISG3106: Rolled steels for welded structure) is used in the form where acorrosion prevention means such as coating is applied to the steelmaterial. “dm” means decimeter.

With respect to coating, a coating film is deteriorated with time sothat periodic maintenance and repair become necessary. Further, it isalso necessary to take into account the sharp increase in labor cost andthe difficulty in recoating. Due to such reasons, recently, a steelmaterial which can be used in a paintless state is required, and thereexists a strong demand for a steel material which can be used in apaintless state.

In view of such current circumstances, recently, as a steel materialwhich can be used in a paintless state in a high air-borne saltenvironment such as a coastal environment, a steel material whichcontains various alloy elements, particularly, a large amount of Ni hasbeen developed.

For example, in Japanese Patent No. 3785271, a high atmosphericcorrosion resistance steel material to which Cu and 1 mass % or more ofNi are added as an element which enhances atmospheric corrosionresistance is disclosed. In Japanese Patent No. 3846218, a steelmaterial having excellent atmospheric corrosion resistance to which 1mass % or more of Ni and 1 mass % or more of Mo are added is disclosed.

In Japanese Patent No. 3466076, a weathering steel material to which Tiis added in addition to Cu and Ni is disclosed. Further, in patentdocument 4, a steel material for welded structures which contains alarge amount of Ni and further contains Cu, Mo, Sn, Sb, P and the likeis disclosed.

However, when the content of Ni is increased as described in JapanesePatent No. 3785271, Japanese Patent No. 3846218 and Japanese Patent No.3466076, there arises a drawback that the cost of the steel material isincreased due to the increase in cost of the alloy.

With the steel material described in JP-A-10-251797 where the content ofNi and P are increased and the steel material contains Cu, Mo, Sn, Sband the like, the cost of the steel material is increased due to theincrease in cost of the alloy, and weldability is lowered due to thehigh content of P.

It could therefore be helpful to provide a steel material which can bemanufactured at a low cost and has the excellent atmospheric corrosionresistance.

SUMMARY

We thus provide:

A steel material having excellent atmospheric corrosion resistance, thesteel material having the composition which contains, by mass %, morethan 0.06% and less than 0.14% C, 0.05% or more and 2.00% or less Si,0.20% or more and 2.00% or less Mn, 0.005% or more and 0.030% or less P,0.0001% or more and 0.0200% or less S, 0,001% or more and 0.100% or lessAl, 0.10% or more and 1.00% or less Cu, 0.10% or more and 0.65% or lessNi, 0.001% or more and 1.000% or less Mo, preferably 0.005% or more and1.000% or less Mo, 0.005% or more and 0.200% or less Nb, and Fe andunavoidable impurities as a balance.

The steel material having excellent atmospheric corrosion resistancedescribed above, wherein the steel material further contains one, two ormore kinds selected from a group consisting of, by mass %, 0.2% or moreand 1.0% or less Cr, 0.01% or more and 1.00% or less Co, 0.0001% or moreand 0.1000% or less REM and 0.005% or more and 0.200% or less Sn.

The steel material having excellent atmospheric corrosion resistancedescribed above, wherein the steel material further contains one, two ormore kinds selected from a group consisting of, by mass %, 0.005% ormore and 0.200% or less Ti, 0.005% or more and 0.200% or less V, 0.005%or more and 0.200% or less Zr, 0.0001% or more and 0.0050% or less B and0.0001% or more and 0.0100% or less Mg.

Our structural steel material having excellent atmospheric corrosionresistance can be acquired at a low cost.

The structural steel material compositely contains elements effectivefor the enhancement of atmospheric corrosion resistance and hence, thestructural steel material can be manufactured at a low cost withoutcontaining a large amount of an expensive element such as Ni, haspractical weldability, and has excellent atmospheric corrosionresistance in a high salt environment such as a coastal environment.Particularly, our steel materials can acquire outstanding effects in ahigh air-borne salt environment where an amount of air-borne saltexceeds 0.05 mdd.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing conditions and cycles of a corrosiontest.

DETAILED DESCRIPTION

We extensively studied the composition of a steel material from aviewpoint of atmospheric corrosion resistance in a high saltenvironment. As a result, we found that by allowing base steel whichcontains Cu and Ni to further compositely contain Mo and Nb, theatmospheric corrosion resistance of the steel material in a high saltenvironment can be enhanced.

Reasons for limiting respective constitutional factors are explainedhereinafter.

1. Composition

First, reasons for restricting the composition of steel are explained.Here, % of all components means mass %.

-   C: more than 0.06% and less than 0.14%

C is an element which enhances the strength of a structural steelmaterial. The structural steel material is required to contain more than0.06% of C to ensure the predetermined strength. On the other hand, whenthe structural steel material contains 0.14% or more of C, weldabilityand toughness of the structural steel material are deteriorated.Accordingly, the content of C is set to a value of more than 0.06% toless than 0.14%. The content of C is preferably 0.08% or more from aviewpoint of ensuring the strength of the structural steel material, andthe content of C is more preferably less than 0.10% from a viewpoint ofweldability and toughness of the structural steel material.

-   Si: 0.05% or more and 2,00% or less

The structural steel material is, as a deoxidizing agent at the time ofsteel making and also as an element to enhance the strength of astructural steel material to ensure the predetermined strength, requiredto contain 0.05% or more of Si. On the other hand, when the structuralsteel material excessively contains Si exceeding 2.00%, toughness andweldability of the structural steel material are remarkablydeteriorated. Accordingly, the content of Si is 0.05% or more to 2.00%or less. The content of Si is preferably 0.10% or more to 0.80% or less.

-   Mn: 0.20% or more and 2.00% or less

Mn is an element which enhances the strength of a structural steelmaterial. The structural steel material is required to contain 0.20% ormore of Mn for ensuring the predetermined strength. On the other hand,when the structural steel material excessively contains Mn exceeding2.00%, toughness and weldability of the structural steel material aredeteriorated. Accordingly, the content of Mn is 0.20% or more to 2.00%or less.

The content of Mn is preferably 0.20% or more to 1.50% or less.

-   P: 0.005% or more and 0.030% or less

P is an element which enhances the atmospheric corrosion resistance of astructural steel material. To acquire such an effect, the structuralsteel material is required to contain 0.005% or more of P. On the otherhand, when the structural steel material contains P exceeding 0.030%,weldability of the structural steel material is deteriorated.Accordingly, the content of P is 0.005% or more to 0.030% or less. Thecontent of P is preferably 0.005% or more to 0.025% or less.

-   S: 0.0001% or more and 0.0200% or less

When a structural steel material contains S exceeding 0.0200%,weldability and toughness of the structural steel material aredeteriorated. On the other hand, when the content of S is lowered to avalue less than 0.0001%, a manufacturing cost is increased. Accordingly,the content of S is 0.0001% or more to 0.0200% or less. The content of Sis preferably 0.0003% or more to 0.0050% or less.

-   Al: 0.001% or more and 0.100% or less

Al is an element necessary for deoxidization at the time of steelmaking. To acquire such an effect, a structural steel material isrequired to contain 0.001% or more of Al. On the other hand, when thecontent of Al exceeds 0.100%, weldability of the structural steelmaterial is adversely influenced. Accordingly, the content of Al is0.001% or more to 0.100% or less. The content of Al is preferably 0.010%or more to 0.050% or less, The content of Al is measured in terms ofacid-soluble Al,

-   Cu: 0.10% or more to 1.00% or less

Cu forms a dense rust layer by making rust particles fine, and has aneffect of enhancing atmospheric corrosion resistance of a structuralsteel material. Such an effect can be acquired when the content of Cu is0.10% or more. On the other hand, when the content of Cu exceeds 1.00%,the cost is increased along with the increase of a consumption amount ofCu. Accordingly, the content of Cu is 0,10% or more to 1.00% or less.The content of Cu is preferably 0.20% or more to 0.50% or less.

-   Ni: 0.10% or more and 0.65% or less

Ni forms a dense rust layer by making rust particles fine, and has aneffect of enhancing atmospheric corrosion resistance of a structuralsteel material. To sufficiently acquire such an effect, the structuralsteel material is required to contain 0.10% or more of Ni. On the otherhand, when the content of Ni exceeds 0.65%, the cost is increased alongwith the increase of a consumption amount of Ni. Accordingly, thecontent of Ni is 0.10% or more to 0.65% or less. The content of Ni ispreferably 0,15% or more to 0.50 or less.

-   Mo: 0.001% or more and 1.000% or less

Mo is an important factor and, by coexisting with Nb, acquires an effectof remarkably enhancing atmospheric corrosion resistance of a steelmaterial in a high salt environment. Further, Mo forms molybdic acidions in a rust layer thus preventing chloride ions which are a corrosionaccelerating factor from permeating the rust layer and reaching the baseiron. Further, MoO₄ ²⁻ is dissolved in the steel material along with ananode reaction of the steel material so that the compound containing Mois precipitated on a surface of the steel material whereby an anodereaction of the steel material is suppressed. To sufficiently acquiresuch an effect, the structural steel material is required to contain0.001% or more of Mo. On the other hand, when the content of Mo exceeds1.000%, the cost is increased along with the increase of a consumptionamount of Mo. Accordingly, the content of Mo is 0.001% or more to 1.000%or less. The content of Mo is preferably 0.005% or more to 1.000% orless, and is more preferably 0.10% or more to 0.70% or less.

-   Nb: 0.005% or more and 0.200% or less

Nb is an important factor and, by coexisting with Mo, acquires an effectof remarkably enhancing atmospheric corrosion resistance of a steelmaterial in a high salt environment. Nb is concentrated in a rust layerin the vicinity of a surface of the steel material thus acquiring aneffect of suppressing an anode reaction of the steel material. Tosufficiently acquire such an effect, the structural steel material isrequired to contain 0.005% or more of Nb. On the other hand, when thecontent of Nb exceeds 0.200%, the toughness of the steel isdeteriorated. Accordingly, the content of Nb is 0.005% or more to 0.200%or less. The content of Nb is preferably 0.010% or more to 0.030% orless.

Although the basic composition of the steel material is as describedabove, when the enhancement of some desired properties of the structuralsteel material is further desired, the structural steel material mayfurther contain one, two or more kinds selected from a group consistingof Cr, Co, REM and Sn as selective elements.

-   Cr: 0.2% or more and 1.0% or less

Cr forms a dense rust layer by making rust particles fine so that Cr iseffective to enhance atmospheric corrosion resistance of a structuralsteel material. When the structural steel material contains 0.2% or moreof Cr, the structural steel material can acquire such an effect, whilewhen the structural steel material contains Cr exceeding 1.0%, theweldability of the structural steel material is deteriorated.Accordingly, when the structural steel material contains Cr, the contentof Cr is preferably 0.2% or more to 1.0% or less. The content of Cr ismore preferably 0.2% or more to 0.7% or less.

-   Co: 0.01% or more and 1.00% or less

Co is distributed in the whole rust layer, and forms a dense rust layerby making rust particles fine so that Co is effective to enhanceatmospheric corrosion resistance of a structural steel material. Whenthe structural steel material contains 0.01% or more of Co, thestructural steel material can acquire such an effect, while when thestructural steel material contains Co exceeding 1.00%, the cost isincreased along with the increase of a consumption amount of Co.Accordingly, when the structural steel material contains Co, the contentof Co is preferably 0.01% or more to 1.00% or less. The content of Co ismore preferably 0.10% or more to 0.50% or less.

-   REM: 0.0001% or more and 0.1000% or less

REM is distributed in a whole rust layer, and forms a dense rust layerby making rust particles fine so that REM is effective to enhanceatmospheric corrosion resistance of a structural steel material. Whenthe structural steel material contains 0.0001% or more of REM, thestructural steel material can acquire such an effect, while when thestructural steel material contains REM exceeding 0.1000%, such an effectis saturated. Accordingly, when the structural steel material containsREM, the content of REM is preferably 0.0001% or more to 0.1000% orless. The content of REM is more preferably 0.0010% or more to 0.0100%or less.

-   Sn: 0.005% or more and 0.200% or less

Sn is concentrated in a lower layer of a rust layer so that Sn iseffective to suppress an anode reaction of the steel material. When thestructural steel material contains 0.005% or more of Sn, the structuralsteel material can acquire such an effect, while when the structuralsteel material contains Sn exceeding 0.200%, the toughness of the steelmaterial is deteriorated. Accordingly, when the structural steelmaterial contains Sn, the content of Sn is preferably 0.005% or more to0.200% or less. The content of Sn is more preferably 0.010% or more to0.100% or less.

Further, the structural steel material can contain one, two or morekinds selected from a group consisting of Ti, V, Zr, B and Mg asselective elements.

-   Ti: 0.005% or more and 0.200% or less

Ti is an element effective to enhance the strength of the steelmaterial. When the structural steel material contains 0.005% or more ofTi, the structural steel material can acquire such an effect, while whenthe structural steel material contains Ti exceeding 0.200%, thetoughness of the steel material is deteriorated. Accordingly, when thestructural steel material contains Ti, the content of Ti is preferably0.005% or more to 0.200% or less. The content of Ti is more preferably0.010% or more to 0.100% or less.

-   V: 0.005% or more and 0.200% or less

V is an element effective to enhance the strength of the structuralsteel material. When the structural steel material contains 0.005% ormore of V, the structural steel material can acquire such an effect,while when the structural steel material contains V exceeding 0.200%,such an effect is saturated. Accordingly, when the structural steelmaterial contains V, the content of V is preferably 0.005% or more to0.200% or less. The content of V is more preferably 0.010% or more to0.100% or less.

-   Zr: 0.005% or more and 0.200% or less

Zr is an element effective to enhance the strength of the structuralsteel material. When the structural steel material contains 0.005% ormore of Zr, the structural steel material can acquire such an effect,while when the structural steel material contains Zr exceeding 0.200%,such an effect is saturated. Accordingly, when the structural steelmaterial contains Zr, the content of Zr is preferably 0.005% or more to0.200% or less. The content of Zr is more preferably 0.010% or more to0.100% or less.

-   B: 0.0001% or more and 0.0050% or less

B is an element necessary to enhance the strength of the structuralsteel material. However, when the content of B is less than 0.0001%, thestructural steel material cannot sufficiently acquire such an effect. Onthe other hand, when the content of B exceeds 0.0050%, the toughness ofthe structural steel material is deteriorated. Accordingly, when thestructural steel material contains B, the content of B is preferably0.0001 or more to 0.0050% or less. The content of B is more preferably0.0005% or more to 0.0040% or less.

-   Mg: 0.0001% or more and 0.0100% or less

Mg is an element effective to enhance toughness of a welded heataffected zone by fixing S in steel. When the structural steel materialcontains 0,0001 or more of Mg, the structural steel material can acquiresuch an effect, while when the structural steel material contains Mgexceeding 0.0100%, an amount of inclusions in the steel is increasedthus deteriorating the toughness of the structural steel material to thecontrary. Accordingly, when the structural steel material contains Mg,the content of Mg is preferably 0,0001% or more to 0.0100% or less. Thecontent of Mg is more preferably 0.0005% or more to 0,0030% or less.

The balance of the structural steel material other than theabove-mentioned components is constituted of Fe and unavoidableimpurities. As the unavoidable impurities, the structural steel materialcan contain 0.010% or less of N and 0.010% or less of O. Further, whenCa which the structural steel material contains as an unavoidableimpurity is present in a large amount in steel, the toughness of awelded heat affected zone is deteriorated. Accordingly, the content ofCa is preferably 0.0010% or less.

2. Manufacturing Condition

A steel material having excellent atmospheric corrosion resistance ismanufactured such that steel having the above-mentioned composition isproduced in a molten state, a slab is obtained by usual continuouscasting or ingot making, and a steel material such as a steel plate, ashaped steel, a steel plate or a bar steel is manufactured byhot-rolling the slab. Heating and rolling conditions are suitablydetermined corresponding to required material quality, and thecombination of such a manufacturing method with controlled rolling,accelerated cooling or heat treatment such as reheating is alsopossible.

EXAMPLE 1

Steel having the chemical composition shown in Table 1-1 and Table 1-2was produced in a molten state, a slab was heated up to 1150° C. and,thereafter, hot rolling was performed, and the rolled plate was cooledto a room temperature by air cooling thus manufacturing a steel platehaving a thickness of 6 mm for a test. Then, a test specimen having asize of 35 mm×35 mm×4 mm was sampled from the obtained steel plate. Thetest specimen is subjected to grinding processing such that a surface ofthe test specimen has surface roughness Ra of 1.6 μm or less. Edge facesand a back side were sealed with a tape, and a front side was alsosealed with a tape such that an area of a front-side exposed areabecomes 25 mm×25 mm.

An atmospheric corrosion resistance evaluation test was performed withrespect to the specimens obtained in the above-mentioned manner, and theatmospheric corrosion resistance of the specimens was evaluated.

In the atmospheric corrosion resistance evaluation test, a corrosiontest which simulates an environment of the inside of a girder having norain shielding members was performed, wherein such an environment isconsidered as the severest environment for the structure such as anactual bridge. The corrosion test was performed by repeating atemperature/humidity cycle in a state where salt adheres to surfaces ofthe above-mentioned specimens.

As shown in FIG. 1, one cycle of a temperature/humidity cycle was set to24 hours in total, wherein a dry step where a temperature is 40° C. anda relative humidity RH is 40% was performed for 11 hours and, then, atransition time was set to 1 hour and, thereafter, a wet step where atemperature is 25° C. and a relative humidity RH is 95% was performedfor 11 hours and, then, a transition time was set to 1 hour. Thetemperature/humidity cycle simulates the temperature/humidity cycle ofan actual environment.

Before starting the temperature/humidity cycle and after every 7 timesof temperature/humidity cycle, artificial seawater solution was appliedto a surface of the test specimen by dropping before the dry step suchthat a quantity of salt adhered to the surface of the test specimenbecomes 1.4 mg/dm².

Under such conditions, the test of 182 temperature/humidity cycles wasperformed over 26 weeks.

After the corrosion test was finished, 1 L(liter) of a rust removingsolution was prepared by adding distilled water to 500 mL of 37%hydrochloric acid, 3.5 g of hexamethylene tetramine and 3 mL of HIBIRON(inhibitor produced by AICOH LTD.). The test specimen was immersed inthe rust removing solution to remove rust and, thereafter, a weight ofthe test specimen was measured. The measurement of the weight wascarried out in accordance with a method described in a materialdelivered in the 145^(th) corrosion prevention symposium “Theacquisition of high accuracy in a method of evaluating the reduction ofcorrosion wear”. An average plate thickness reduction amount on onesurface of a test specimen was calculated by obtaining the differencebetween an obtained weight and an initial weight and by dividing thedifference in weight by an area of a test subject surface of the testspecimen.

The environment where an amount of air-borne salt is approximately 0.5mdd corresponds to an environment where an amount of air-borne salt islarge such as a coastal environment. It is understood from the findingmade heretofore that a steel plate thickness reduction amount (182 days)in this corrosion test is substantially equal to a steel plate thicknessreduction amount by corrosion when the specimen was exposed to an actualenvironment where an amount of air-borne salt is approximately 0.5 mddfor 182 days.

Further, in obtaining a corrosion amount after 100 years based on anaverage plate thickness reduction amount obtained by the test byextrapolation, assuming that the average plate thickness reductionamount obtained during the period of this corrosion test is 22 μm orless, the average plate thickness reduction amount after 100 years isexpected to be 0.5 mm or less which means no occurrence of a laminarpeeling rust.

In general, it is known that whether or not a paintless weathering steelis applicable to a bridge is determined based on whether or not theplate thickness reduction amount after 100 years is 0.5 mm or less.Accordingly, this corrosion test is performed with respect to variouskinds of steel materials, and if an average plate thickness reductionamount obtained by the corrosion test is 22 μm or less, the paintlessweathering steel is applicable to a bridge.

Based on such understanding, as shown in Table 1-1 and Table 1-2, it isdetermined that the steel materials where the average plate thicknessreduction amount is 22 μm or less have excellent atmospheric corrosionresistance.

The result of the corrosion test obtained from above is shown in Table1-1 and Table 1-2 together with the composition.

As shown in Table 1-1 and Table 1-2, in steel kinds No. 1 to 17 and No.32 to 37 which are our examples, the plate thickness reduction amount is19.7 to 22.0 μm, that is, all plate thickness reduction amounts of oursteel kinds are 22 μm or less so that our examples have excellentatmospheric corrosion resistance.

On the other hand, with respect to the steel kinds No. 18 to No. 31which are comparison examples, the steel kinds No. 18 to No. 24 do notcontain one or more kinds selected from a group consisting of Cu, Ni, Moand Nb which are indispensable components of our steel material. Thecontent of Cu is less than a lower limit in the steel kind No. 25. Thecontent of Mo is less than a lower limit in the steel kinds No. 26 andNo. 29. The content of Nb is less than a lower limit in the steel kindNo. 27. The content of Ni is less than a lower limit in the steel kindNo. 28. The content of Sn is less than a lower limit in the steel kindNo. 30. The content of Nb is less than a lower limit in the steel kindNo. 31. Accordingly, in those steel kinds, a plate thickness reductionamount is 24. 3 to 30.7 μm which exceeds 22 μm and hence, it isunderstood that the atmospheric corrosion resistance of the comparisonexamples is greatly inferior to the atmospheric corrosion resistance ofour examples.

TABLE 1-1 Steel Composition (mass %) kind Sol No. C Si Mn P S Al Cu NiMo Nb Cr Co REM  1 0.100 0.21 0.92 0.014 0.0016 0.030 0.25 0.40 0.4140.020 — —  2 0.097 0.18 0.85 0.016 0.0026 0.020 0.27 0.31 0.157 0.012 —— —  3 0.091 0.22 0.80 0.012 0.0015 0.025 0.25 0.15 0.402 0.010 — — —  40.085 0.19 0.73 0.013 0.0022 0.021 0.35 0.25 0.504 0.031 — — —  5 0.0970.17 0.81 0.015 0.0025 0.038 0.23 0.44 0.594 0.037 — 0.310 —  6 0.0850.18 0.75 0.013 0.0025 0.027 0.39 0.23 0.356 0.042 — — 0.0046  7 0.0950.18 0.87 0.015 0.0034 0.040 0.40 0.31 0.093 0.025 — — —  8 0.085 0.220.78 0.017 0.0017 0.023 0.33 0.43 0.673 0.023 — — —  9 0.096 0.18 0.760.015 0.0030 0.019 0.30 0.34 0.434 0.019 — — — 10 0.082 0.19 0.73 0.0120.0025 0.025 0.41 0.33 0.706 0.017 — — — 11 0.097 0.21 0.93 0.014 0.00160.036 0.32 0.24 0.862 0.036 — — — 12 0.097 0.23 0.79 0.022 0.0029 0.0400.36 0.49 0.302 0.057 — — — 13 0.083 0.23 0.92 0.021 0.0018 0.031 0.390.41 0.641 0.011 0.690 — — 14 0.087 0.19 0.87 0.020 0.0028 0.041 0.320.19 0.233 0.018 — 0.451 — 15 0.090 0.22 0.80 0.019 0.0025 0.021 0.220.48 0.268 0.023 0.487 0.217 0.0067 16 0.083 0.22 0.78 0.014 0.00290.033 0.21 0.29 0.126 0.052 — — 0.009  17 0.083 0.23 0.75 0.013 0.00330.027 0.37 0.41 0.579 0.076 — 0.240 — Plate thickness Steel reductionkind Composition (mass %) amount No. Sn Ti V Zr B Mg (μm) Remarks  1 — —— — — — 21.2 present invention example  2 — — — — — — 21.7 presentinvention example  3 — — — — — — 21.2 present invention example  4 — — —— — — 20.9 present invention example  5 — — — — — — 21.1 presentinvention example  6 — — — — — — 21.4 present invention example  7 0.096— — — — — 21.0 present invention example  8 — 0.056 — — — — 21.1 presentinvention example  9 — — 0.087 — — — 21.3 present invention example 10 —— — 0.066 — — 20.9 present invention example 11 — — — — 0.0031 — 20.8present invention example 12 — — — — — 0.0022 21.2 present inventionexample 13 — — — — — — 20.7 present invention example 14 0.044 — — — — —20.3 present invention example 15 — — — — — — 20.1 present inventionexample 16 0.120 — — — — — 20.0 present invention example 17 0.074 — — —— — 19.7 present invention example Note: Underlined parts indicatevalues outside the scope of the present invention. criterion: 22 μm orless

TABLE 1-2 Steel Composition (mass %) kind Sol No. C Si Mn P S Al Cu NiMo Nb Cr Co REM 18 0.095 0.19 0.76 0.017 0.0019 0.022 — — — — — — — 190.088 0.20 0.88 0.020 0.0031 0.021 — 0.15 — — — — — 20 0.099 0.21 0.930.020 0.0037 0.038 — — — 0.035 — — — 21 0.081 0.18 0.85 0.020 0.00380.034 0.54 — — — — — — 22 0.084 0.18 0.81 0.022 0.0028 0.019 — — 0.358 —— — — 23 0.081 0.19 0.76 0.021 0.0037 0.036 0.44 0.42 — — — 0.119 — 240.094 0.24 0.90 0.013 0.0031 0.037 — 0.23 0.261 0.011 — — — 25 0.0820.22 0.85 0.013 0.0016 0.023 0.05 0.36 0.478 0.015 — — — 26 0.096 0.230.72 0.016 0.0017 0.037 0.15 0.19 — 0.053 — — — 27 0.080 0.24 0.69 0.0140.0021 0.026 0.22 0.13 0.231 0.002 — — — 28 0.099 0.21 0.77 0.014 0.00280.027 0.36 0.06 0.167 0.024 0.233 — — 29 0.073 0.22 1.50 0.014 0.00290.033 0.27 0.29 — 0.012 — — — 30 0.073 0.22 1.50 0.014 0.0029 0.033 0.270.29 0.001 0.012 — — — 31 0.073 0.22 1.50 0.014 0.0029 0.033 0.27 0.290.001 0.003 — — — 32 0.073 0.22 1.50 0.014 0.0029 0.033 0.27 0.29 0.0010.012 — — — 33 0.060 0.19 1.40 0.012 0.0029 0.031 0.21 0.31 0.001 0.013— — — 34 0.078 0.25 1.30 0.010 0.0028 0.021 0.30 0.35 0.001 0.018 — — —35 0.073 0.22 1.50 0.014 0.0029 0.033 0.27 0.29 0.003 0.012 — — — 360.073 0.22 1.50 0.014 0.0029 0.033 0.27 0.29 0.007 0.012 — — — 37 0.0730.22 1.50 0.014 0.0029 0.033 0.27 0.29 0.102 0.012 — — — Plate thicknessSteel reduction kind Composition (mass %) amount No. Sn Ti V Zr B Mg(μm) Remarks 18 — — — — — — 30.7 comparison example 19 — — — — — — 28.7comparison example 20 — — — — 0.0022 — 29.5 comparison example 21 — — —— — — 27.6 comparison example 22 — — — 0.041 — — 27.3 comparison example23 — — — — — — 25.4 comparison example 24 0.041 — — — — — 24.6comparison example 25 — — — — — — 24.9 comparison example 26 — 0.016 — —— — 25.1 comparison example 27 — — 0.045 — — — 24.4 comparison example28 — — — — — 0.0045 24.9 comparison example 29 0.050 — — — — — 24.8comparison example 30 0.003 — — — — — 24.3 comparison example 31 0.050 —— — — — 25.2 comparison example 32 0.050 — — — — — 22.0 presentinvention example 33 0.035 — — — — — 21.8 present invention example 340.044 — — — — — 21.7 present invention example 35 0.050 — — — — — 21.8present invention example 36 0.050 — — — — — 21.3 present inventionexample 37 0.050 — — — — — 20.5 present invention example Note:Underlined parts indicate values outside the scope of the presentinvention. criterion: 22 μm or less

1. A steel material having a the composition which contains, by mass %,more than 0.06% and less than 0.14% C, 0.05% or more and 2.00% or lessSi, 0.20% or more and 2.00% or less Mn, 0.005% or more and 0.030% orless P, 0.0001% or more and 0.0200% or less S, 0.001% or more and 0.100%or less Al, 0.10% or more and 1.00% or less Cu, 0.10% or more and 0.65%or less Ni, 0.001% or more and 1.000% or less Mo, 0.005% or more and0.200% or less Nb, and Fe and unavoidable impurities as a balance. 2.The steel material according to claim 1, further containing, by mass %,0.005% or more and 1.000% or less Mo.
 3. The steel material according toclaim 1, further containing one, two or more kinds selected from thegroup consisting of, by mass %, 0.2% or more and 1.0% or less Cr, 0.01%or more and 1.00% or less Co, 0.0001% or more and 0.1000% or less REMand 0.005% or more and 0.200% or less Sn.
 4. The steel materialaccording to claim 1, further containing one, two or more kinds selectedfrom the group consisting of, by mass %, 0.005% or more and 0.200% orless Ti, 0.005% or more and 0.200% or less V, 0.005% or more and 0.200%or less Zr, 0.0001% or more and 0.0050% or less B and 0.0001% or moreand 0.0100% or less Mg.
 5. The steel material according to claim 2,further containing one, two or more kinds selected from the groupconsisting of, by mass %, 0.2% or more and 1.0% or less Cr, 0.01% ormore and 1.00% or less Co, 0.0001% or more and 0.1000% or less REM and0.005% or more and 0.200% or less Sn.
 6. The steel material according toclaim 2, further containing one, two or more kinds selected from thegroup consisting of, by mass %, 0.005% or more and 0.200% or less Ti,0.005% or more and 0.200% or less V, 0.005% or more and 0.200% or lessZr, 0.0001% or more and 0.0050% or less B and 0.0001% or more and0.0100% or less Mg.
 7. The steel material according to claim 3, furthercontaining one, two or more kinds selected from the group consisting of,by mass %, 0.005% or more and 0.200% or less Ti, 0.005% or more and0.200% or less V, 0.005% or more and 0.200% or less Zr, 0.0001% or moreand 0.0050% or less B and 0.0001% or more and 0.0100% or less Mg.